In the field of imaging, document files can include vector graphic elements which represent the intended appearance of a document. Some documents include graphic elements that overlap such that the overlapping colors combine to produce a different color.
A document has a native format, such as a document produced by word processing, artwork, or photo software. A document can also be translated into a portable format that can be used by viewing, editing, or printing software. Printing systems typically receive portable documents which may be processed prior to printing by a target printing device. Processing may include many steps including, for example, preflight, color matching and trapping.
Portable documents can be in a variety of formats, such as raster formats and vector formats. Vector formats, exemplified by Adobe® PDF or Adobe® PostScript®, define a document in terms of an ordered list of graphic elements that represent a page image based on attributes defined by an imaging model (e.g. opaque and transparent). When printing a PDF document for example, elements are painted in order on the page to create a rendering of the document. Documents can have many pages with sets of elements representing the intended image for each page. For simplicity, the remainder of the description assumes a file (or stream of data) for a document, the file including graphic elements representing an intended image of a single page of the document.
Exemplary graphic element types include text, path (shape), and image (raster) types. The color painted for a document at any point (e.g. pixel in the rendered image) is affected by elements configured to paint there, the element painting order, the element attributes and the imaging model. In some cases (e.g. elements configured to knockout color), the element painting last defines the color. Other cases (e.g. elements configured to overprint color) are more interesting in that more than one element may contribute color. This is particularly relevant when two elements overlap at the point of interest. Color space, overprint and transparency attributes for overlapping elements can also, for example, affect the color at the overlap point.
Commonly-assigned U.S. Patent Application Publication No. 2005/0280847 (Cairns et al.) describes a method for processing document files including overlapping elements configured to overprint, and is incorporated by reference herein. In summary, the prior application discloses converting a document by retaining original elements while adding a graphic element to the document to represent the combined color of elements that overlap in the area. The application also discloses processing the converted file to, for example, alter the color to match a target device's color space.
While this approach produced the desired results, there is more complexity than desired in calculating the overlapped overprint areas and combining colors.